I don't know that much about gun customization, but I think that you can take certain parts of some guns and attach them to other guns, for example taking the stock off a sniper rifle and putting it on a shotgun. I'm not fully certain about it but I would like that. I do know that guns are heavily customizable, being able to change every thing from the skin to the ammo counter. I not sure if you fully change every gun in the game, seeing as Bungie would want their exotic weapons to keep their own appearances. Now about vehicles, I know that the hover bike that all players get get be customized, but I don't how much it can be customized. Also I heard in a weekly update that you could use swords in the game, I'm not really sure about that but it would pretty fun. It would be awesome if could customize swords though.